Electron beam pattern generator

ABSTRACT

An electron beam pattern generator for the rapid and accurate generation of electron images upon a surface such as a surface coated with an electron sensitive resist material. The pattern generator is comprised of a field emission electron gun, the emission of which is focused and deflected by deflection coils controlled by a computer through digital to analog converters. Blanking of the beam is accomplished by deflection of the beam off range and onto the edge of an aperture plate. A stable high current beam is achieved by heating the field emission electrode. A properly biased device collects the secondary emission from the object on which the electron pattern is focused, and together with the deflection signals for the beam may be used to create a CRT display. A means for achieving step and repeat capability is also disclosed, as is a means for maintaining constant exposure dosage independent of beam current changes.

United States Patent 1 on 3,922,546

Livesay l l Nov. 25, 1975 [54l ELECTRON BEAM PATTERN GENERATOR A Simple Scanning Electron Microscope," by Crewe et al., from Review of Scientific Instruments, Vol. 40, [75] Inventor, \CrVa|lIiI;am R. Llvesay, Camarillo, N0, Feb 1969 Pp 241-246- I Automatic Pattern Positioning of Scanning Electron [73] Asslgnge: Rad'am Energy s Exposure," by Miyauchi et al.. from IEEE Transac- Newbury Park Calif tions on Electron Devices, Vol. EDI7. No. 6, June [221 Filed: Mar. 5, 1974 1970. PP 50- 5 [2H Appl' 448283 7 Primary Examiner-Davis L. Willis Related US. Application Data Attorney, Agent, or FirmFulwider Patton, Rieber. [63] Continuation-impart of Ser. No. 244,078, April l4, Lee & UtEChl I972v abandoned.

[57] ABSTRACT [52] US. Cl. 250/310; 250/398; 250/440 I [51] lm. Cl} G01N 23 00 An electron beam pattern generator for the rapid and [58] Field of Se h 250/306 307 310 39 accurate generation of electron images upon a surface 250/397. 398 399 400 439 440 442; such as a surface coated with an electron sensitive re 219 12 EB sist material. The pattern generator is comprised of a field emission electron gun, the emission of which is [56} References Cited focused and deflected by deflection coils controlled by UNITED STATES PATENTS a computer through digital to analog converters I Blanking of the beam is accomplished by deflection of gjzaankist...i 219/!2 EB the beam off range and onto the edge of an aperture 3 10/1972 Baldwij 'g 250/306 plate. A stable high current beam is achieved by heat- OTHER PUBLICATIONS A High Resolution ElectronBeam System for Micro circuit Fabrication," by Chang et al., from Record of the 10th Symposium on Electron, Ion and Laser Beam Technology, Gaithersburg, Md, USA, May 2l-23, 1969, pp. 97-106.

Electron Gun Using a Field Emission Source, by Crewe et al., from Review of Scientific Instruments, Vol. 39, No. 4, Apr. 1968, pp. 576-583.

ing the field emission electrode. A properly biased device collects the secondary emission from the object on which the electron pattern is focused, and together with the deflection signals for the beam may be used to create a CRT display. A means for achieving step and repeat capability is also disclosed, as is a means for maintaining constant exposure dosage independent of beam current changes.

23 Claims, 5 Drawing Figures flew [WAYS/ON 7/7 A PEI? Tu/rE :304 506 EFAF X Pos/T/o/v SENSOR Y Po SIT/ON Sewsaa U.S. Patent Nov. 25, 1975 Sheet 3 of4 3,922,546

ELECTRON BEAM PATTERN GENERATOR This is a Continuation-In-Part application of my previously filed application for same, as Ser. No. 244,078 and filed Apr. 14, I972, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of electron image pattern generators, and particularly to such pattern generators for use in semiconductor device fabrication.

2. Prior Art The fabrication of present day semiconductor devices utilizes a series of processing steps which require the printing of a highly detailed and accurate image on a sensitized surface on a semiconductor wafer. By way of example various regions are diffused into a semiconductor wafer by applying a sensitized coating to the oxide layer over the wafer, exposing the coating through a mask so as to print the desired circuit image thereon, developing the coating and dissolving away the developed or undeveloped portion, etching the oxide layer in the regions exposed as a result of development of the coating, dissolving away the remainder of the developed coating, diffusing the desired impurities into the exposed regions of the semiconductor substrate and reoxidizing the exposed surface of the substrate. The printing of the image on the sensitized surface is accomplished by a contact printing process, with the image side of the mask flat against the sensitized surface of the wafer so as to reproduce the mask image on the sensitized surface as accurately as possible. Consequently, abrasion and physical deterioration of the mask result, thereby limiting the number of times a mask may be used before it must be replaced. The cost of replacement of such masks contributes significantly to the cost of the resulting integrated circuit.

Typically the masks used for semiconductor fabrication are comprised of a metal, metal halide film on a glass plate which is exposed to the desired pattern and developed in a manner similar to any photograph negative. Thus, to fabricate such a mask, the desired pattern for the mask is created by hand on a much expanded scale and with a contrasting background. This pattern is then photographed and subsequently reduced to the desired size by a series of photo reduction steps. Also, at some stage in the photo and photo reduction processes, a film is repeatedly exposed to the pattern in a matrix layout by the use of a step and repeat camera whereby, as a result thereof, the one pattern, typically on the order of one tenth of an inch square, is repeated perhaps hundreds of times on the mask blank. Development of the mask blank completes the mask, though typically such a mask may be used as a master for the production of other masks to allow replacement of production type masks without undergoing a step and repeat or photoreduction processes for the new masks.

It is apparent from the above description that the conventional process of mask fabrication has a number of problems. The original layout of the circuit is subject to inaccuracy and human error which may go undetected until devices are actually fabricated with the resulting mask. Similarly, any change to the mask requires a change or a new layout, and a repeat of all of the mask fabricating steps. Also, limitations in the equipment used such as the optical equipment used in the photoreduction process and the step and repeat 2 equipment result in a limitation upon the achievable details in the mask, typically limiting the size of individual component in the integrated circuit.

The desirability of directly creating a mask image of the desired size has been recognized, and electron image pattern generating equipment has been built to directly expose the electron sensitized mask blank to an electron image generated by such systems. These systems utilize a conventional electron gun creating an electron beam from the thermionic emission of a heated filament. The emission is limited by various apertures, focused by magnetic lenses, and deflected by deflection coils externally controlled to create the desired image. This system however, has not heretofore found wide acceptance because of certain limitations therein. These limitations include:

1. Limited scan area of high resolution. To achieve small diameter electron beams requires the placement of the mask blank a relatively small working distance from the final electron lens. This short working distance limits the scan area due to deflection distortion and deflection defocusing which are both functions of the deflection angle.

2. Slow scanning speed in conventional electron optical columns using magnetic lenses. The deflection coils are located in the bore of the iron core of the lenses. Thus, final lens deflection speed is slowed by flux linkages and eddy currents created in the area surrounding the deflection coils, causing a lag in beam position with respect to the deflection signal.

3. Limited cathode life in systems using thermionic cathodes, cathode life is typically under hours. This is due to the boiling off of cathode material and contamination of the emitting surface.

4. Hysteresis. Due to the fact that iron core lenses are used in the vicinity of the deflection coils, inaccuracy in scan position are introduced do to hysteresis in the magnetic deflection system. This error, of course, is in addition to the scanning speed error of No. 2 above.

5. Low brightness at large scan angles. To achieve large scan angles and provide for coverage and pattern generation over large areas requires a decrease in the final aperture angle to minimize deflection defocusing and distortion. In addition, since spherical aberration for a given lens increases with an increase in working distance, the aperture must be stopped down to main tain a small beam size for writing high resolution patterns. The result of this decrease in aperture angle at large working distances is a decrease in beam current in a given size beam, thus necessitating much longer times to generate a given pattern. Of course, the time required to generate a pattern is a most important parameter for such equipment, since this will largely determine the productivity thereof.

There is therefore needed an electron image pattern generation system which will provide an accurate and rapidly controllable, relatively high current electron beam, together with a system for providing a step and repeat capability and for allowing computer control of the beam to directly generate masks of very high quality and resolution.

BRIEF SUMMARY OF THE INVENTION An electron beam pattern generator for the rapid and accurate generation of electron images upon a surface such as a surface coated with an electron sensitive resist material. The pattern generator is comprised of a field emission electron gun. the emission of which is focused through an electrostatic lens and subsequently deflected by deflection coils controlled by a computer through digital to analog converters. Blanking of the beam is accomplished through a separate blanking coil also controlled by the computer by the deflection of the beam off range and onto the edge of an aperture plate. A stable high current beam is achieved by heating the field emission electrodes. preferably in range of 500 to 600C. A properly biased device. which may be in essence an electron multiplier tube, collects the secondary emission from the object on which the electron pattern is focused. and together with the deflection signals for the beam may be used to create a cathode ray tube display. The object on which the electron beam pattern is generated is mounted on an X-Y table accurately controllable through computer control by suitable control circuitry. Inaccuracy in the actual position of the table is detected by suitable detectors and may be used either to provide a correction to the deflection coil sig nals to minutely deflect the beam in accordance with the true position of the table, or. provided a suitable table is used. to provide for the servo control of the table to the true desired position within the required accuracy. Also exposure dosage is maintained constant. even in the presence of exposure beam current fluctuations. by maintaining a constant exposure beam current-exposure time product.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of the field emission electron gun system of the present invention Electron Beam Pattern Generator.

FIG. 2 is a block diagram of the electron beam pattern generator system of the present invention.

FIG. 3 is a block diagram similar to FIG. 2 and further illustrating the exposure control system.

FIG. 4 is a partial cross-sectional view of the scintillator light pipe combination for collecting secondary electrons.

FIG. 5 is a circuit diagram of the exposure control system.

DETAILED DESCRIPTION OF THE INVENTION First referring to FIG. 1, a schematic diagram of the field emission electron gun system 20 of the present invention Electron Beam Pattern Generator may be seen. The pattern generator components within the enclosure 22 are generally housed within a chamber which may be evacuated to a relatively high vacuum, preferably in the range of 5 X l to X torr or better. A pointed field emission tip 24 is generally disposed at the top of the chamber 22 and generally aligned with the aperture in the electrostatic lens, comprised of members 26 and 28. A tip voltage power supply 30 negatively biases the tip 24 with respect to the members 26 of the electrostatic lens so as to cause a steady rate of field emission from the tip and to accelerate the emitted electrons toward the electrostatic lens. Field emission. of course, is to be distinguished from thermionic emission in that infield emission, a high electric field in the vicinity of the emitter lowers the surface potential barrier so that appreciable numbers of electrons can penetrate the barrier by quantum mechanical tunneling. The emitted currents can be quite large because all of the surface directed electrons in the metal participate. rather than just those occupying states in the tail of the Fermi distribution. as in thermionic emission.

Anodes 26 and 28 are biased and disposed so as to effectively focus the field emission from the tip 24, with the total electron accelerating voltage being the voltage of the voltage supply 32 connected between the tip 24 and anodes 28. In a preferred embodiment, anodes 28 as well as most other components within the chamber 22 are maintained at ground potential so that the volt age between the tip and anodes 28 is the final accelerating voltage for the electrons in the electron beam.

Typically the field emission tip 24, generally a tungsten tip. is maintained at a high negative potential and the first anode 26 is biased positive relative to the tip by perhaps 2,000 volts. This creates a high field about the tip which causes field emission to take place. The sec ond anode is at ground potential. and by adjusting the ratio of the tip voltage to the accelerating voltage. a focusing of the tungsten tip at some point beyond the second anode 28 is accomplished. The size of the image of the tip produced by the gun is given by the equation where d, is the diameter of the tip, d the focused spot size. m the magnification of the gun, V0 the initial emission energy of electrons leaving the tip, and V, 1 the electrostatic potential applied to the tip. Thus. with a tip size d, of approximately 5,000 angstroms, a magni-- fication m of I, an initial emission energy of electrons Vo of 0.2 electron volts, and an electrostatic potential of the tip of 2,000 volts, the spot size of the beam pro duced will be approximately 50 angstroms. Such electrostatic focusing means are well known in the prior art, and have been used. by way of example, in the CWICKSCAN electron microscope manufactured by Coates and Welter Instrument Corporation, Sunnyvale, California, a subsidiary of American Optical Con poration.

The opening in the electrostatic lens is approximately a 600 micron aperture, and mounted therebelow is a further aperture 34 having an aperture of approxi mately 200 microns to further limit the beam size. Mounted below the last aperture is a set of deflection coils 36 for electromagnetic deflection of the beam. generally indicated by numeral 38, as the beam passes through the magnetic field established by the deflection coils. Deflection coils 36 are adapted to deflect the beam in an X direction in proportion to an electrical signal applied (schematically) on lind 4t], and to deflect the beam in the line direction proportional to a signal applied on line 42. Thus, independent X and Y control of the electron beam is achieved.

Also mounted on the deflection coil assembly 36 is an additional coil responsive to an electrical signal ap plied on line 44. This coil is also disposed so as to deflect the beam but is intended not for proportional deflection but for gross deflection to deflect the beam out of range, more specifically to deflect the beam so as to cause the beam to be intercepted by blanking aperture 46 mounted between the deflection coils and the item on which the beam is being generally directed (eg the target). Thus, a single on-off type signal may be applied on line 44 to direct the beam off range and therefore blank the signal. The advantage of using a separate blanking coil as opposed to using the deflection coils is that the deflection coils and drive circuitry therefore are carefully designed so as to provide the high degree of accuracy and linearity required, and further as shall subsequently be seen, are designed to operate in response to a digital signal from a digital control means, typically a digital computer. Thus, the circuitry for the primary deflection coils tends to have a longer response time than a simple on-off driver for the blanking coil. Consequently, use of a separate blanking coil allows total blanking of the beam without interferring with the X-Y beam deflection signal and allows rapid recovery of the beam to the desired X-Y position upon termination of the blanking signal. Also, in this regard, it should be noted that deflection caused by the blanking coil should be sufficient to deflect the beam from any X-Y position in range to an off range position so that blanking is assured, independent of the deflection signal applied on lines 42.

The beam when not deflected off range, projects downward through the blanking aperture 46 and secondary electron collecting device 50 onto the surface of the object 52 on which the pattern is generated, principally a photomask blank. The object or target 52 is mounted on an X-Y table 54 which may be controllably and accurately caused to step or move in the X and Y directions in response to command signals applied on lines 56 and 58.

When the electron beam strikes the target, a number of secondary electrons are emitted from the target which may even exceed the number of electrons in the electron beam. These secondary electrons are gener ally relatively low energy electrons and are emitted in a generally diffuse pattern, as opposed to being directionally oriented. By biasing the collecting surface of device 50 slightly positive with respect to the target area, these secondary electrons will be collected by device 50 to provide a current on line 60 dependent upon the number of secondary electrons emitted. (The device 50 preferably is an electron multiplier device, and the collecting surface may, by way of example, be the first plate of a multiplier tube). This provides an instantaneous measure of the secondary electrons emitted, which may be combined with the X-Y deflection coil information so as to be displayable on a cathode ray tube to provide a grossly magnified image of the pattern being generated during generation, or to scan an exposed and developed electron resist coated surface to quickly display the developed image (e.g. a scanning electron beam microscope mode).

As an alternate embodiment, back scatter sensors may be used in place of or in addition to the secondary emission device 50 to detect back scatter from the target. Such detectors are characterized as having an electron energy threshold sensitivity so as to sense high energy electrons, such as reflected electrons, but not low energy electrons such as secondary electrons. For certain applications this alternate has advantages, and provides a different basis of contrast in the image displayed on the CRT display. For this purpose photodiodes may be used, such as are manufactured by Solid State Radiations, Inc., Los Angeles, California (their PIH series such as the ()l0-PIN-T05, 050-PIN-T08 etc., diodes). To maximize the signal derived, the diodes should be placed to intercept as many reflected electrons as possible.

The advantage of using field emission phenomena in the pattern generator of the present invention are manyfold. Since no magnetic lenses are required, there is substantially no hysteresis in scanning, thus resulting in faster scanning speeds than in thermionic emissions.

Also, the accuracy in beam positioning obtainable is greater due to the fact that there is no hysteresis and there are fewer apertures which could become contaminated and charged to deflect the beam in an uncontrolled and undesired manner. Also, using field emission allows a larger scan area, that is, due to the small beam diameter produced by the field emission gun. it can be projected through longer working distances, thereby minimizing distortion and deflection defocusing since the beam will cover large areas with smaller deflection angles. Further, the field emission gun has a much longer cathode life of up to 1,000 hours emission lifetime compared to l0 to I00 hours for conventional thermionic cathodes. Also, the brightness or current densities achievable using field emission are relatively high so that the time required to generate a given pat tern with the system of the present invention can be an order of magnitude shorter than for a conventional system using a thermionic cathode.

To further assist in maintenance of a stable, high current density beam using a field emission tip. it has been found advantageous to heat the emitting tip. This tends to drive off contaminants which might otherwise inhibit or increase emission and cause uncontrolled and undesired drifts or fluctuation in the beam current densities. Thus, a tip heater power supply 62 is provided to apply electrical power to the heater for the emitting tip 24. In the preferred embodiment, the tip 24 is simply affixed to a supporting wire member 64 and the tip heater power supply is adapted to pass the current through the supporting wire so as to cause heating thereof to the desired temperature. It has been found that both stability and current density may be enhanced if the tip is heated to within a range of approximately 300 to 800C, and preferably in the range of 500 to 600C higher temperatures will cause substantial thermionic emission, and even heating the tip to the stated range results in better stability at high current densities at the expense of increased chromatic aberration. However, because of the small spot size obtainable some deterioration thereof may be tolerated. (If an even smaller spot size is desired, an electrostatic or magnetic lens may be provided below the field emission gun to provide further demagnification of the beam for even higher resolution of microfabrication).

The chamber 22 in the preferred embodiment is provided with the vacuum interlock system so that the target item 52 may be inserted into the vacuum chamber or removed from the vacuum chamber without requiring the exposure of the inside of the vacuum chamber to atmospheric pressure and contamination, and without requiring prolonged pump down times. This eliminates the major cause of contamination in such systems and decreases the cycle time for processing target objects through the system. Such interlocks are well known in the prior art.

It is of course difficult to very accurately place the target object on the X-Y table, a problem which is made even more difficult as a result of the pass-through ports or interlocks previously mentioned. Also, since the system will generally be used in a step and repeat mode to generate photomasks having a matrix of smaller mask patterns thereon which must be very accurately located with respect to each other, the step and repeat accuracy of the system is potentially limited by the step and repeat positioning accuracy of the X-Y table. It has been found that one effective method of overcoming this last problem is to utilize a table which may be readily controllable to within a reasonable accuracy, together therewith position sensors to very accurately sense the true position of the table and to provide correction signals to the X-Y beam deflection signals to deflect the beam slightly in accordance with the true position of the table. Thus, correction signals are provided to correct the beam position in accordance with the physical inaccurancy of the commanded table position. These signals may be provided on lines 66 and 68 and may be combined with the command signals of the X and Y deflection to provide the desired correction.

Various devices may be used to accurately detect the true table positions as hereabove described. One such system which has been used is a laser interferometer, as are now commercially available. Such a device may be used to accurately determine the table position throughout any reasonable range of table motion. However, such devices are expensive and since two axes of information (X and Y) are required, two such interferometers must be used, thereby substantially increasing the overall cost of the system.

Another means of accurately sensing the table position may be provided by providing holographic phase gratings on each axis and using optical sensors to detect the position of the fixed gratings with respect to the moving gratings on the table. Such a system is also highly accurate and is relatively inexpensive. Also, it should be noted that although in the preferred embodiment, the signals on lines 66 and 68 indicating the true position of the table are used to provide correction signals to the beam deflection coils, these signals may also be applied to the table control means, depending upon the nature of the table used, so as to essentially servo the table to the true desired position (tables having sliding parts, etc., may tend to exhibit substantial static friction, thereby making servo control of extremely small table motions to correct the table position difficult).

Having now described the electron gun and support ing apparatus, the overall system shall be described with reference to FIG. 2. The basic control for the system is provided by a digital computer 70 of conventional design operating under program control, and input means 72 is provided for providing to the computer the basic data for the pattern to be generated. (In the preferred embodiment, a Nova 800 Computer, manufactured by Data General, lnc., Southboro, Mass, is used). The input means. also of conventional design (some of which are also available from Data General) may utilize punched cards, punched tape. magnetic tape and the like for storage of the desired information to be read into the computer, either entirely before pattern generation commences, r progressively as pattern generation is carried out so as to reduce the required memory capacity of the computer. Of course. in gen eral point by point data information will not be provided to the computer but instead the computer will be programmed to decifer coded and simplified input in formation to generate the output signals required. By way of specific example a subroutine may be provided for controlling the electron beam to generate a rectan gular area, and such subroutine called into play by an input (or as a result of a previous input stored in the computer) which identifies the X and Y coordinates for each corner of the rectangular area and commands the execution of the subroutine using the coordinants as a reference. In this way, relatively complicated patterns 8 may be rapidly and accurately generated under com puter control using computer input information in a relatively simple form. (Also, it should be noted that the electron beam may be significantly defocused by varying the tip voltage on the emission tip 24, so that a controllably larger beam spot size on the target surface may be achieved. Thus, as an alternate embodiment the digital computer may also provide an output signal so as to control the tip voltage so as to be capable of drawing lines or filling in areas by utilizing an enlarged spot size. Of course, a larger spot size generally means a reduced beam current density so that a slower scan rate is generally required to adequately expose the electron sensitive material, all of which of course is readily achieved under the digital computer program control).

The primary outputs of the digital computer, preferably presented in bit parallel form are as schematically shown in FIG. 2, specifically an X deflection control a Y deflection control, a blanking signal, and X table control and a Y table control. The X and Y deflection control in preferred embodiment are 16 bit signals which gives a resolution of plus or minus one half in 2 or approximately plus or minus one half in 65,000. Thus, for a scan area as large as 0.2 inches by 0.2 inches, each bit represents a beam step of approximately three microinches, thereby providing the desired resolution and beam position accuracy.

The X and Y digital deflection signals are converted to analog deflection signals by the 16 bit digital to analog converters 74 and 76. The output of the D to A converters are applied to the deflection coils through lines 40 and 42, with the X-Y table position correction signals on lines 66 and 68 being added thereto by the suming networks 78 and 80 to provide a deflection correction in accordance with the true position of the X-( table. The X and Y analog deflection signals on line 40 and 42 may also be coupled to a cathode ray tube deflection control circuit 82 which in turn drives the X and Y deflection device in cathode ray tube display 84 in unison with the X-Y scan of the electron beam on the target surface 52.

Beam blanking is accomplished by a blanking signal generator which may accept a one bit signal to generate an on-off blanking signal on line 44. The computer also presents the X and Y signals for the X-Y table control through controllers 88 and 90 which are coupled to the X-Y table in the pattern generator 20. The nature of these last digital signals and the signals derived therefrom will, of course, depend upon the specific type of X-Y tables used, since any relatively accurate and easily controllable X-Y table is well suited for use in this system provided it is suitable for use in a vacuum environment. Finally, the signal from the device 50 indicating that the magnitude of the secondary emission from the target is applied through line 60 to the cathode ray tube beam density control circuit 86 which controls the beam intensity in the cathode ray tube 84 to provide the contrast for the pattern areas. (The blanking signal need not be applied to the cathode ray tube display directly since blanking is effectively sensed by a signal on line 60, indicating an absence of secondary electrons being emitted from the target area.

In the fabrication of masks utilizing the present invention, it is important that the exposure dosage for the exposed electron resist be uniform throughout the mask area and also be uniform from mask to mask. This assures not only a maximum rate of mask generation consistent with adequate exposure of the electron resist, but further assures repeatable line widths and edge definition as excessive dosage will lead to secondary effects resulting in significant exposure of the electron resist beyond the intended field of the beam. Obviously one way this may be achieved is to maintain the exposure beam current constant by suitable control of the electron emitter (a field emission tip in the present invention). This,however is not easily accomplished, since the ratio of exposure beam current to total beam current varies with time, so that the desired result may not be achieved by merely maintaining total beam current constant. Accordingly, another approach is to measure the exposure beam current and to control the electron emitter so as to vary the total beam current as necessary to maintain the constant exposure beam current. This however is not easily achieved, since the field emission tip is typically at a voltage of about 10,000 volts or more with respect to the target focusing coils, etc., and in addition the voltage fluctuations required on the field emission tip to achieve this result are sufficient to create problems because of the capacitive coupling between the tip and other structure. Accordingly, in the present invention the exposure dosage is maintained substantially constant not by maintaining the exposure beam current constant, but by maintaining the product of the exposure beam current and the exposure time constant.

Now referring to FIG. 1, the manner of detecting the exposure beam current may be seen. A scintillator light pipe photo-multiplier combination is utilized as a sensor for sensing the secondary electrons emitted by the target. (In the fabrication of masks the emission of secondary electrons as well as back scattered electrons is constant over the entire area, as the electron resist thickness will be uniform throughout the area of the mask, and in turn will be disposed over a uniform masking layer such as silicon, chromium and the like, depending on the type of mask being fabricated). This detector, also shown in part in FIG. 4, is comprised of a collector 100 and a light pipe 102 coupled to a photomultiplier 104, providing a photomultiplier output signal on line 106. The periphery of the end 101 of light pipe 102 is provided with a high bias voltage, with the end area 110 covered with a scintillator material so that secondary electrons collected by a relatively low bias voltage on the collector 100 are sufficiently accelerated to impact the scintillator material with a sufficient velocity to cause a light emission having an intensity proportional to the electron collection rate, which in the case of photomask fabrication is directly proportional to the exposure beam current. Thus the photo multiplier tube output is proportional to the beam current. This scintillator light pipe photo-multiplier arrangement has been used in scanning electron microscopes by the Coates and Welter instrument Corpora tion, Sunnyvale, California, with the scintillator being identified by that company as their Endural Electron Detector.

Now referring to FIG. 5, a block diagram of the circuit used to process the photo-multiplier tube output to provide the signal desired to maintain constant beam current may be seen. The output of a photomultiplier tube on line 106 is inverted and buffered by amplifier A8, and scaled by amplifier Al. The output of amplifier Al is coupled to the positive input of operational amplifier A2 and to the negative input of operational amplifier A3, utilized as an inverting amplifier. Accordingly, the input to the positive input of operational amplifier A4 on line I08 is the inverse of the input to operational amplifier A2 on line 110. The output of amplifiers A4 and A2 are coupled through resistors R1 and R2 respectively to the bases of transistors Tl and T2 respectively. The emitters of transistors TI and T2 are coupled through resistors R3 and R4 respectively to positive and negative power sources +VS and -VS respectively. The collectors of transistors T1 and T2 are coupled to opposite points of a diode bridge formed by diodes D1, D2, D3 and D4.

The operation of amplifiers A4 and A2 together with transistors T1 and T2 may be illustrated as follows: When the output of the photomultiplier 104 increases the voltage on line 108 decreases. Assuming the gain of amplifier A4 is very high the negative input thereto on line 112 must equal the voltage on line 108. Accordingly, l3==(VS-Vll2) R3. However, V112 is responsive to the output of a photomultiplier tube on line 106. However, it will be noted that the signal on line 106 is not the only signal applied to the input of amplifier A1.

In particular, in addition to the coupling of signal on line 107 through an adjustable resistor P1 and a series resistor R5, there is also coupled a signal from a reference voltage tVR through a resistor R6 (a feedback resistor R7 is also included to complete the circuit for amplifier A1, it being understood that feedback resistors for the other amplifiers have been omitted for pur poses of explanation only and for clarity in the circuit in the circuit drawing). Resistors R6 and P1 are used for sealing and to set the proper bias level for the circuit.

It will be assumed that the output of a photo-multiplier tube is a positive going signal. (A negative going signal may be readily inverted by an inverting amplifier not shown). Accordingly, the output of the photo-multiplier tube may be expressed as some constant (K) times the exposure beam current leb. The connection the resistor R6 to a reference voltage VR provides an additional signal input K through amplifier Al. Accordingly, the total input to amplifier A] may be expressed as --K,,l,,,+l(,. This same signal, after gain changes, appears on line 108 and may be expressed as K l,,,+K the new constants reflecting the new scaling of the signal. As before, assuming amplifier A4 is a high gain amplifier, the same signal will appear on line 2 as voltage V112. Accordingly, combining these equations there results:

It will be noted that by proper selection of the parameters, K3 may be made to equal VS, in which case KIIQD Accordingly, the current [3 may be made directly proportional to the exposure beam current I. Similarly I4 is directly proportional to the exposure beam current and ideally, by matching components, l4 should be equal to [3.

Line 114 is coupled to the junction between diodes DI and D3, while line 116 is coupled to the junction 124 between diodes D2 and D4, and is further coupled to ground through a capacitor C1. Line 116 is also coupied to the negative input of operational amplifier A5, with the output being coupled to line 114 and through resistor R8 to a network comprising resistor R9 coupled between ground and the positive input of amplifier A5, and a series combination of capacitor C2 and resistor R10 also coupled between ground and the positive input 122 of amplifier A5. The amplifier A operates as a threshold detector, providing a high level positive output or high level negative output depending upon the differential input thereto. Further, amplifier A5 should be a high speed amplifier, particularly charac terized by a short switching time from high positive and negative outputs to the opposite thereof.

Assuming that the base current in transistors T1 and T2 are small, the current into junction 118 is 13, and the current into junction 120 is 14. Assume for the moment that capacitor C1 has a sufficient charge on it so that the voltage on line 116 exceeds the voltage on line I22. In this case, the output of amplifier A5 on line 114 will be negative, thereby holding junction 124 to a voltage lower than the voltage on junctions 118, 120 and 126. Accordingly, diode D1 will be forward biased and diode D2 will be reverse biased, so that the current I3 flows through diode D1 and line 114. At the same time, diode D3 is reverse biased. Accordingly, current 14 flows through diode D4, thus tending to discharge capacitor Cl and lower the voltage on line 116. When the voltage on line 116 decreases to the value on line 122, the differential input in amplifier A5 reverses and line 114 quickly goes to a high positive output. This changes the voltage on line 122 to an intermediate pos itive value determined by resistors R8, R9 and R10 with capacitor C2 slowly charging so that the voltage on line 22 reasonably quickly approaches a higher value determined by resistors R8 and R9. When line 114 switches positive, diode D3 is forward biased, thereby supplying current 14 therethrough and back biasing diode D4. At the same time diode D1 is back biased so that current 13 flows through diode D2, again charging capacitor C1 until the voltage on line 116 is increased to the diode of voltage on line 122, at which time line 114 again switches negative. It is apparent, therefore, that the voltage appearing on line 114 is a square wave, a positive portion of which results in the discharging of capacitor C1 to a predetermined negative voltage and the negative portion of which results from the charging of capacitor C1 from the predetermined negative voltage to a predetermined positive voltage. Also, since the charging rate is dependent on 13 and 14, both of which are directly proportional to the exposure beam current, it is apparent that the frequency of the square wave appearing on line 114 is directly proportional to the exposure beam current.

The remainder of the circuit comprised of inverting amplifier A5 one shots OS1 and 082, NAND gates N 1, N2 and N3 and NOR gat NORl comprises a frequency doubling and enabling circuit. In particular, terminal 140 is used as a basic enable terminal to provide an enabling signal to the exposure beam current responsive circuit hereinbefore described, whereas terminal 142 may be used as an input for a fixed frequency to provide a fixed frequency output on terminal 144 independent ofthe exposure beam current. When the signal on terminal 140 is in the high state, the output of amplifier of A6 on line 146 is in the low state, thereby disabling NAND gate N2. The voltage on terminal 140 enables NAND gate N1, so that the inverse of the square wave appearing on line 114 is coupled through line 148 to the positive input of one shot OS1 and the negative input of one shot 052. These one shots are single shot or monostable flip flops, with 051 as coupled providing a positive pulse upon the positive going portions of the square wave on line 148, and 082 providing a positive pulse on the negative going portion of the square wave appearing on line 148. The outputs of one shots OS1 and 082 on lines 150 and 152 respectively, are provided as inputs to NOR gate NORl. Accordingly, the output of the NOR gate is in the high state holding the output of NAND gate N3 in the low state until either of the inputs to the NOR gates are pulsed to the high state. Accordingly, assuming NAND gate N3 has enabled the signal appearing on line 144, the output on line 144 will be a pulse train having one pulse for each positive going shift of voltage on line 114 and one pulse for each negative going voltage change on line 114, thereby having a frequency or repetition rate equal to twice that of the square wave on line 114.

As an alternate embodiment, a commercially available voltage controlled oscillator such as the Teledyne Philbriclt VCO No. 4705 may be used in place of the circuit of FIG. 5.

Now referring to FIG. 3, a block diagram illustrating the manner in which the circuit of FIG. 5 is utilized may be seen. In this figure, all elements identified by two digit numerals are elements which have been previously discussed with respect to FIG. 2, and therefore will not be again described except for computer 70, which in the illustration in FIG. 3 has been separated into a computer a and memory 70b. In this embodiment the memory preferably is provided with a direct memory access or data channel access capability; that is, memory access may occur at any time rather than being limited by the computer clock. On the other hand, the computer 70a is intended to continually update the information stored in the memory 70b, so as to be accessible when required to provide information with respect to the location and desired exposure time as well as focus information for the next point in the pattern being generated.

This embodiment may be operated as follows: a computer program is first derived comprising a plurality of subroutines which, on presentation of certain data may be used to generate the point to point information with respect to focus and exposure time to expose an area of a predetermined geometry. By way of example, one subroutine may be to create information regarding a rectangular path based upon certain minimum information such as length, width and coordinants of one corner of the path, another subroutine to create point to point information for circular regions based upon a specified diameter and coordinants for the center thereof, etc. Such a program would be used for all mask generation, with data for each individual mask being provided for use with that program in the form of se quential identification of basic region location and geometry for use with the subroutines which, when taken together specifies the entire pattern to be generated. While blanking is not required when stepping from one geometrical region to a connected geometrical region, blanking may still be used routinely in such stepping so that no control is required over the area to be swept out by the beam when sweeping from the finish point of one geometry to the start point of a connected region. (It has been found that very responsive blanking may be achieved by the confirmation of an aperture 300 with a first grounded plate 302 and a second control plate 304 above the electrostatic lens. A negative blanking signal on line 306 is preferred, as this deflects the beam away from the aperture with less tendency for the beam to intercept either plate to possibly release contamination, etc.) Accordingly, for the first geometrical region to be drawn by the electron beam the computer would provide memory 70b with data on the first few successive points to be exposed, e.g. XY coordinants in digital form, a focus control and an exposure count. (The memory, therefore, is merely a register or storage means to provide, preferably on command, data in digital form for each successive point in the pattern to be generated.) Information for the first point is then used to provide a signal to the focus control 200 to focus or controllably defocus the beam, so as to sweep out a relatively wide or narrow line (the drawing of a pad area may be achieved either by a focused beam controlled in close steps of short duration to sweep up the area, or by a defocused beam stepped in greater steps at a slower rate to expose the region). Also, for the first point, the memory provides not only the XY deflection signals, e.g. point coordinants to the deflection controls 74 and 76, but also provide an exposure count to set the count down counter 204. Of course, prior to the stabilization of the X and Y deflection signals, a signal would be provided to the blanking generator to blank out the beam until the deflection system had settled. Exposure of the first point then proceeds with circuit 200 providing a count down frequency to counter 204 which is proportional to the exposure beam current. When the count down counter 204 reaches zero, an advance signal is provided on line 206 to the memory 70b to provide data regarding the next point to be exposed to the focus control, the deflection control and the count down counter. No blanking is required between successive points of a given geometrical region, as the subroutine would control the drawing of the region so that the beam never progresses beyond the limits of the intended region.

It is apparent from the foregoing explanation that the beam steps from point to point to fill in the particular region. in practice, at least for a focused beam, the exposure times may be as low as a single count from the count down counter with the time required for the advance of the beam being substantial in comparison with the individual exposure time. Consequently, in that case beam motion is something between a point to point exposure and a smooth sweep along a predetermined line. Of course, the computer 70a is used to con stantly update the information in memory 70b, so that data on the next point is always available when needed. It is apparent also that should the exposure beam current decrease, the rate at which the count down counter will in fact count through the preset number for each point will decrease, so that the exposure beam current exposure time product will remain constant. Finally, when stepping to the next geometrical area to be exposed, whether connected to the preceding area or not, a blanking signal will first be provided to blank the exposure beam, the coordinants of the next area will be presented to the X and Y deflection controls, and the focus control signal will be provided to the focus control. After a predetermined length of time sufficient to insure settling of the various circuits as may be controlled by the computer clock, the blanking signal is removed and the point by point exposure of the next region proceeds. Accordingly it is seen that the net result is the accurate point to point exposure of a predetermined pattern with a total exposure of each por tion of the pattern closely controlled within limits rcgardless of any reasonable variations in the exposure beam current, with the result that a high degree of accuracy and repeatability may be achieved through the present invention system on a day to day basis, even though the characteristics of the field emission tip exhibits both short and long term drifts or a replacement tip has substantially different characteristics. Of course the step and repeat characteristics are achieved upon the completion of one pattern by again blanking the beam, advancing the XY table under computer control, relocating the beam and repeating the pattern generation at the new matrix location to create a matrix of patterns characteristic of masks used in integrated circuit manufacture.

Thus, there has been described herein an electron beam pattern generator system using field emission principles and adapted to operate under computer control for rapid and accurate exposure of the electron sensitive surface in the desired pattern. The system has a number of advantages over prior art systems, including greater accuracy, greater speed and higher current densities in the beam, thereby resulting in a more efficient, productive and useful pattern generator. Also, while the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

1 claim:

1. In an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field;

means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target;

means for focusing said beam onto said target;

deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals;

blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking sig nal; and

means for heating said emitting means substantially continuously, thereby improving long-term stability of said beam.

2. The apparatus of claim 1 wherein said focusing means is an electrostatic lens.

3. The apparatus of claim 2 wherein said deflection means and said blanking means are electromagnetic deflection means.

4. The apparatus of claim 1 further comprised of a secondary electron collecting and detecting means, said last named means being disposed adjacent the surface of said target and biased to accelerate secondary electrons emitted from said target to said last named means, and further being a means for providing an electrical signal output responsive to the number of secondary electrons collected.

S. The apparatus of claim 1 wherein said target support means is a means for moving said target in two generally orthogonal directions approximately perpendicular to said beam in response to control signals applied thereto.

6. The apparatus of claim 5 further comprised of po' sition detecting means for accurately detecting the position of said target and providing output signals rcsponsivc to the error between said position of said target and the position indicated by said control signals applied to said target support means.

7. The apparatus of claim 6 further comprised of means for combining said output signals of said posi tion detecting means with said control signals for said deflection means to provide control signals for said deflection means which are altered in accordance with said error.

8. The apparatus of claim 1 further comprised of a cathode ray tube and a secondary electron collecting and detecting means, said last named means being disposed adjacent the surface of said target and biased to accelerate secondary electrons emitted from said target to said last named means. and further being a means for providing an electrical signal output responsive to the number of secondary electrons collected, said cathode ray tube being coupled to said last named means and said control signals for each deflection means whereby said cathode ray tube maybe caused to display an electron beam pattern being generated on said target.

9. In an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

electron emitting means for emitting electrons and directing said electrons in a beam toward said tar get;

deflection means for deflecting said beam in response to deflection control signals;

control means for providing deflection control signals to said deflection means; and

sensing means for sensing the current of said beam impinging on said target, said sensing means being coupled to said control means and being operative to advance the position of said beam at a rate dependent upon said current of said beam.

10. The apparatus of claim 9 wherein said sensing means is a means including a photomultiplier tube for sensing secondary electrons emitted by said target.

11. The apparatus of claim 9 further comprised of a means for focusing said beam. said control means being coupled to said means for focusing to control the focus ofsaid beam in cooperation with the control of said deflection control signals 12. A method of controlling the exposure ofa target in an electron beam pattern generator comprising the steps of:

a. directing an electron beam toward the target b. sensing the current in the beam impinging on the target; and

c4 advancing the beam at a rate dependent upon the beam current.

13. The method of claim 12 wherein step (b) is ac complished by sensing the secondary electrons emitted by the target.

14. The method of claim 12 wherein said beam is advanced at a rate substantially proportional to beam current.

15. The method of claim 12 wherein step (c) comprises advancing the beam in a point to point manner at 16 a rate dependent upon the beam current and focus and controlling the focus at each point.

16. In an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field;

means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target;

means for focusing said beam onto said target;

deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals; blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking signal; sensing means for providing a signal responsive to the beam current impinging on said target; and

control means coupled to said sensing means for advancing the beam position by control of said deflection means in response to said beam current impinging on said target.

17. The apparatus of claim 16, wherein said control means is a means for advancing the beam position at a rate substantially proportional to said beam current.

18. The apparatus of claim 17, wherein said sensing means includes a photomultiplier tube for collecting and sensing secondary electrons emitted from said target.

19. In an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field;

means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target;

means for focusing said beam onto said target;

deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals;

blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking sigrial;

sensing means for providing a signal responsive to the beam current impinging on said target, said sensing means including a photomultiplier tube for collecting and sensing secondary electrons emitted from said target; and

control means coupled to said sensing means for advancing the beam position by control of said deflection means in a manner responsive to said beam current impinging on said target, said control means being a means for advancing the beam position at a rate substantially proportional to said beam current, and said control means including an oscillator means responsive to the output of said photomultiplier tube to provide an output fre quency substantially proportional to said beam cur- 1 7 rent.

20. The apparatus of claim 19, wherein said control means is further comprised of a storage means and a counter means, said storage means being coupled to said deflection means and being a means for storage of control information for the next desired beam position, said counter means being coupled to the output of said photomultiplier tube and said storage means and being a means for providing an advance signal to said storage means upon a resettable count, said storage means further being a means for presenting said control information to said deflection means and resetting said counter means upon the occurrence of said advance signal.

21. In an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field;

means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target;

means for focusing said beam onto said target;

deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals; and

blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking signal, said blanking means including an aperture means for providing an aperture between said electron emitting means and said means for focusing, and a blanking control means adjacent said beam and between said electron emitting means and said aperture means, said blanking control means being a means for deflecting said beam away from the aperture defined by said aperture means by the control of the voltage on said blanking control means,

22. In an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

electron emitting means for emitting electrons and directing said electrons in a beam toward said target;

deflection means for deflecting said beam in response to deflection control signals;

control means for providing deflection control signals to said deflection means; and

sensing means for sensing the current of said beam impinging on said target, said sensing means being coupled to said control means and being operative to advance the position of said beam at a rate dependent upon said current of said beam;

and wherein said control means includes a storage means for storage in digital form of the coordinates for at least first and second successive beam positions, said storage means being responsive to an advance signal to advance its output from the coordinates of said first beam position to said second beam position. the output of said storage means being coupled to and controlling said deflection means, and said control means further including an oscillator means and a counter means, said oscillator means being coupled to said sensing means and said counter means, and being a means for providing a signal to said counter means which has a frequency dependent on the current in said beam, said counter means being coupled to said storage means and being a means for providing said advance signal upon the occurrence of a preset count.

23. ln an electron beam pattern generator, the apparatus comprising:

means for supporting a target;

electron emitting means for emitting electrons and directing said electrons in a beam toward said tar- 8 deflection means for deflecting said beam in response to deflection control signals;

control means for providing deflection control signals to said deflection means;

sensing means for sensing the current of said beam impinging on said target, said sensing means being coupled to said control means and being operative to advance the position of said beam at a rate dependent upon said current of said beam;

means for focusing said beam, said control means being coupled to said means for focusing to control the focus of said beam in cooperation with the control of said deflection control signals;

aperture means for defining an aperture between said electron emitting means and said means for focus ing; and

blanking means disposed adjacent to said beam between said electron emitting means and said aperture means, said blanking means being responsive to a blanking signal to deflect said beam away from said aperture means.

i IR l PATENT NO.

DATED iNVENTOR(S) I November 25,

1975 lliam R. Livesay It is certified that error appears in the above-identified patent and that said Letters Pii -wi are hereby corrected as shown below:

Column Column Column Column Column Column Column Column Column [SEAL] line line line line line line line lines line 3, "component" should be components 37-38, "inaccuracy should be inaccuracies 38, "do should be due 61, after "that" insert an 6l, "infield" should be in field 50, "lind" should be line 67, "therefore" should be therefor 8, "interferring" should be interfering 2, "therewith" should be with 8, inaccurancy" should be inaccuracy 33-34, "suming" should be summing 62, "area." should be area.)

10, line 24, "tVR" should be +V line line line line line line

28, delete "in the circuit"; 37, after "connection" insert of 38, "VR" should be V s 50-52, "VS" in equations (two occurences) should be V 56, "gat" should be gate "200" should be 202 thirtieth D a); 9f March 1976 A ttesr:

RUTH C. MASON Arresting Officer C. MARSHALL DANN ('ummz'ssimur ufParenrs and Trademarks 

1. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field; means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target; means for focusing said beam onto said target; deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals; blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking signal; and means for heating said emitting means substantially continuously, thereby improving long-term stability of said beam.
 2. The apparatus of claim 1 wherein said focusing means is an electrostatic lens.
 3. The apparatus of claim 2 wherein said deflection means and said blanking means are electromagnetic deflectioN means.
 4. The apparatus of claim 1 further comprised of a secondary electron collecting and detecting means, said last named means being disposed adjacent the surface of said target and biased to accelerate secondary electrons emitted from said target to said last named means, and further being a means for providing an electrical signal output responsive to the number of secondary electrons collected.
 5. The apparatus of claim 1 wherein said target support means is a means for moving said target in two generally orthogonal directions approximately perpendicular to said beam in response to control signals applied thereto.
 6. The apparatus of claim 5 further comprised of position detecting means for accurately detecting the position of said target and providing output signals responsive to the error between said position of said target and the position indicated by said control signals applied to said target support means.
 7. The apparatus of claim 6 further comprised of means for combining said output signals of said position detecting means with said control signals for said deflection means to provide control signals for said deflection means which are altered in accordance with said error.
 8. The apparatus of claim 1 further comprised of a cathode ray tube and a secondary electron collecting and detecting means, said last named means being disposed adjacent the surface of said target and biased to accelerate secondary electrons emitted from said target to said last named means, and further being a means for providing an electrical signal output responsive to the number of secondary electrons collected, said cathode ray tube being coupled to said last named means and said control signals for each deflection means whereby said cathode ray tube may be caused to display an electron beam pattern being generated on said target.
 9. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; electron emitting means for emitting electrons and directing said electrons in a beam toward said target; deflection means for deflecting said beam in response to deflection control signals; control means for providing deflection control signals to said deflection means; and sensing means for sensing the current of said beam impinging on said target, said sensing means being coupled to said control means and being operative to advance the position of said beam at a rate dependent upon said current of said beam.
 10. The apparatus of claim 9 wherein said sensing means is a means including a photomultiplier tube for sensing secondary electrons emitted by said target.
 11. The apparatus of claim 9 further comprised of a means for focusing said beam, said control means being coupled to said means for focusing to control the focus of said beam in cooperation with the control of said deflection control signals.
 12. A method of controlling the exposure of a target in an electron beam pattern generator comprising the steps of: a. directing an electron beam toward the target b. sensing the current in the beam impinging on the target; and c. advancing the beam at a rate dependent upon the beam current.
 13. The method of claim 12 wherein step (b) is accomplished by sensing the secondary electrons emitted by the target.
 14. The method of claim 12 wherein said beam is advanced at a rate substantially proportional to beam current.
 15. The method of claim 12 wherein step (c) comprises advancing the beam in a point to point manner at a rate dependent upon the beam current and focus and controlling the focus at each point.
 16. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field; means for subjecting said emitting means to a strong electrostatic field to cause field emiSsion therefrom and to accelerate said electrons generally in a beam and toward said target; means for focusing said beam onto said target; deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals; blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking signal; sensing means for providing a signal responsive to the beam current impinging on said target; and control means coupled to said sensing means for advancing the beam position by control of said deflection means in response to said beam current impinging on said target.
 17. The apparatus of claim 16, wherein said control means is a means for advancing the beam position at a rate substantially proportional to said beam current.
 18. The apparatus of claim 17, wherein said sensing means includes a photomultiplier tube for collecting and sensing secondary electrons emitted from said target.
 19. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field; means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target; means for focusing said beam onto said target; deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals; blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking signal; sensing means for providing a signal responsive to the beam current impinging on said target, said sensing means including a photomultiplier tube for collecting and sensing secondary electrons emitted from said target; and control means coupled to said sensing means for advancing the beam position by control of said deflection means in a manner responsive to said beam current impinging on said target, said control means being a means for advancing the beam position at a rate substantially proportional to said beam current, and said control means including an oscillator means responsive to the output of said photomultiplier tube to provide an output frequency substantially proportional to said beam current.
 20. The apparatus of claim 19, wherein said control means is further comprised of a storage means and a counter means, said storage means being coupled to said deflection means and being a means for storage of control information for the next desired beam position, said counter means being coupled to the output of said photomultiplier tube and said storage means and being a means for providing an advance signal to said storage means upon a resettable count, said storage means further being a means for presenting said control information to said deflection means and resetting said counter means upon the occurrence of said advance signal.
 21. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; an electron emitting means for emitting electrons substantially as a result of field emission due to the presence of a strong electrostatic field; means for subjecting said emitting means to a strong electrostatic field to cause field emission therefrom and to accelerate said electrons generally in a beam and toward said target; means for focusing said beam onto said target; deflection means for accurately deflecting said beam within a controllable range in response to deflection control signals; and blanking means for deflecting said beam beyond said controllable range and onto a beam intercepting member to prevent impingement of said beam on said target in response to an electrical blanking signal, said blanking means including an aperture means for providing an aperture between said electron emitting means and said means for focusing, and a blanking control means adjacent said beam and between said electron emitting means and said aperture means, said blanking control means being a means for deflecting said beam away from the aperture defined by said aperture means by the control of the voltage on said blanking control means.
 22. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; electron emitting means for emitting electrons and directing said electrons in a beam toward said target; deflection means for deflecting said beam in response to deflection control signals; control means for providing deflection control signals to said deflection means; and sensing means for sensing the current of said beam impinging on said target, said sensing means being coupled to said control means and being operative to advance the position of said beam at a rate dependent upon said current of said beam; and wherein said control means includes a storage means for storage in digital form of the coordinates for at least first and second successive beam positions, said storage means being responsive to an advance signal to advance its output from the coordinates of said first beam position to said second beam position, the output of said storage means being coupled to and controlling said deflection means, and said control means further including an oscillator means and a counter means, said oscillator means being coupled to said sensing means and said counter means, and being a means for providing a signal to said counter means which has a frequency dependent on the current in said beam, said counter means being coupled to said storage means and being a means for providing said advance signal upon the occurrence of a preset count.
 23. In an electron beam pattern generator, the apparatus comprising: means for supporting a target; electron emitting means for emitting electrons and directing said electrons in a beam toward said target; deflection means for deflecting said beam in response to deflection control signals; control means for providing deflection control signals to said deflection means; sensing means for sensing the current of said beam impinging on said target, said sensing means being coupled to said control means and being operative to advance the position of said beam at a rate dependent upon said current of said beam; means for focusing said beam, said control means being coupled to said means for focusing to control the focus of said beam in cooperation with the control of said deflection control signals; aperture means for defining an aperture between said electron emitting means and said means for focusing; and blanking means disposed adjacent to said beam between said electron emitting means and said aperture means, said blanking means being responsive to a blanking signal to deflect said beam away from said aperture means. 